Strained semiconductor device with facets

ABSTRACT

A semiconductor device having a substrate including a major surface, a gate stack comprising a sidewall over the substrate and a spacer over the substrate adjoining the sidewall of the gate stack. The spacer having a bottom surface having an outer point that is the point on the bottom surface farthest from the gate stack. An isolation structure in the substrate on one side of the gate stack has an outer edge closest to the spacer. A strained material below the major surface of the substrate disposed between the spacer and the isolation structure having an upper portion and a lower portion separated by a transition plane at an acute angle to the major surface of the substrate.

FIELD

The disclosure relates to integrated circuit fabrication, and more particularly to a semiconductor device with a strained structure.

BACKGROUND

When a semiconductor device, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), is scaled down through various technology nodes, high-k gate dielectric layer and metal gate electrode layer are incorporated into the gate stack of the MOSFET to improve device performance with the decreased feature sizes. In addition, strained structures in source and drain (S/D) recess cavities of the MOSFET utilizing selectively grown silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implement such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. As the gate length and spacing between devices decrease, these problems are exacerbated. For example, it is difficult to achieve an enhanced carrier mobility for a semiconductor device because strained materials can not deliver a given amount of strain into channel region of the semiconductor device, thereby increasing the likelihood of device instability and/or device failure.

Accordingly, what is needed is an improved method for fabricating a strained structure in a semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the relative dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method for fabricating a semiconductor device comprising a strained structure according to various aspects of the present disclosure; and

FIGS. 2-5B show schematic cross-sectional views of a strained structure of a semiconductor device at various stages of fabrication according to various aspects of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

FIG. 1 is a flowchart illustrating a method 100 for fabricating a semiconductor device 200 comprising a strained structure 220 (shown in FIGS. 5A and 5B) according to various aspects of the present disclosure. FIGS. 2-5B show schematic cross-sectional views of a strained structure 220 of a semiconductor device 200 at various stages of fabrication according to an embodiment of the method 100 of FIG. 1. The semiconductor device 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 1 does not produce a completed semiconductor device 200. A completed semiconductor device 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 1 through 5B are simplified for a better understanding of the inventive concepts of the present disclosure. For example, although the figures illustrate the semiconductor device 200, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 wherein a substrate 202 comprising a major surface 202 s is provided. In one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). Further, the substrate 202 may include an epitaxial layer (epi layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

The substrate 202 may further comprise active regions 204. The active regions 204 may include various doping configurations depending on design requirements as known in the art. In some embodiments, the active regions 204 may be doped with p-type or n-type dopants. For example, the active regions 204 may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The active regions 204 may act as regions configured for a N-type metal-oxide-semiconductor transistor device (referred to as an NMOS) and regions configured for a P-type metal-oxide-semiconductor transistor device (referred to as a PMOS).

Isolation structures 206 a, 206 b may be formed on the substrate 202 to isolate the various active regions 204. The isolation structures 206 a, 206 b may utilize isolation technology, such as local oxidation of silicon (LOCOS) or shallow trench isolation (STI), to define and electrically isolate the various active regions 204. In the present embodiment, the isolation structures 206 a, 206 b include a STI. The isolation structures 206 a, 206 b may comprise silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-K dielectric material, other suitable materials, and/or combinations thereof. The isolation structures 206 a, 206 b, and in the present embodiment, the STI, may be formed by any suitable process. As one example, the formation of the STI may include patterning the semiconductor substrate 202 by a conventional photolithography process, etching a trench in the substrate 202 (for example, by using a dry etching, wet etching, and/or plasma etching process), and filling the trench (for example, by using a chemical vapor deposition process) with a dielectric material. In some embodiments, the filled trench may have a multi-layer structure such as a thermal oxide liner layer filled with silicon nitride or silicon oxide. The isolation structure 206 a in the substrate 202 has the outer edge 206 e.

Still referring to FIGS. 1 and 2, the method 100 continues with step 104 in which a gate stack 210 a is formed by sequentially depositing and patterning a gate dielectric layer 212 and a gate electrode layer 214 on the substrate 202. The gate stack 210 a may be formed using any suitable process, including the processes described herein.

In one example, a blanket gate dielectric layer 212 is formed on the substrate 202. In some embodiments, the gate dielectric layer 212 may be a thin film comprising silicon oxide, silicon nitride, silicon oxy-nitride, high-k dielectrics, and/or other suitable dielectric materials. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gate dielectric layer 212 is a high-k dielectric layer with a thickness in the range of about 10 to 30 angstroms. The gate dielectric layer 212 may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric layer 212 may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric layer 212 and substrate 202. The interfacial layer may comprise silicon oxide.

A blanket gate electrode layer 214 is then formed on the blanket gate dielectric layer 212. In some embodiments, the gate electrode layer 214 may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layer 214 may comprise poly-silicon. Further, the gate electrode layer 214 may be doped poly-silicon with the same or different doping. In one embodiment, the gate electrode layer 214 comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 214 may be formed using a suitable process such as low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD), or combinations thereof.

Next, a hard mask layer 216 is formed over the blanket gate electrode layer 214 and a patterned photo-sensitive layer (not shown) is formed on the hard mask layer 216. The pattern of the photo-sensitive layer is transferred to the hard mask layer 216 and then transferred to the gate electrode layer 214 and gate dielectric layer 212 to form gate stacks 210 a, 210 b, and 210 c over the major surface 202 s of the substrate 202. The hard mask layer 216 comprises silicon oxide. Alternatively, the hard mask layer 216 may optionally comprise silicon nitride, silicon oxynitride, and/or other suitable dielectric materials, and may be formed using a method such as CVD or PVD. The hard mask layer 216 comprises a thickness in the range from about 100 to 800 Å. The photo-sensitive layer is stripped thereafter.

A conformal spacer material is then deposited around each of the gate stacks 210 a, 210 b, and 210 c. In the present embodiment, the spacer material may include silicon oxide, silicon nitride, silicon oxy-nitride, or other suitable material. The spacer material may comprise a single layer or multilayer structure. A blanket layer of the spacer material may be formed by CVD, PVD, ALD, or other suitable technique. Then, an anisotropic etching is performed on the spacer material to form a spacer array 218. In the present embodiment, the spacer array 218 adjoining sidewalls 210 s, 210 w of the gate stack 210 a over the substrate 202 is referred to as a pair of spacers 218 a, 218 b. The spacer 218 a comprises a bottom surface 218 s having an outer point 218 p that is the point on the bottom surface 218 s farthest from the gate stack 210 a. The dielectric layer comprises a thickness ranging from about 5 to 15 nm. Further, the isolation structure 206 a in the substrate 202 on one side of the gate stack 210 a has the outer edge 206 e closest to the spacer 218 a.

Referring to FIGS. 1 and 3, the method 100 continues with step 106 in which the substrate 202 is recessed to form source and drain (S/D) recess cavities 220, 230, 240, and 250 adjacent to the gate stacks 210 a, 210 b, and 210 c in the substrate 202. In the structure of FIG. 3, formation of the S/D recess cavities 220, 230, 240, and 250 may be started using a biased dry etching process, followed by an non-biased wet or dry etching process.

In the present embodiment, the substrate 202 is a so-called (001) substrate having a major surface 202 s. Using the spacer array 218 and isolation structures 206 a, 206 b as hard masks, the biased dry etching process is performed to recess the major surface 202 s of the substrate 202 that are unprotected or exposed to form source and drain (S/D) recess regions (not shown), wherein the S/D recess regions may comprise a bottom surface parallel to the major surface 202 s of the substrate 202 and sidewalls perpendicularly to the major surface 202 s of the substrate 202. In an embodiment, the biased dry etching process may be performed under a pressure of about 1 mTorr to 1000 mTorr, a power of about 50 W to 1000 W, a bias voltage of about 20 V to 500 V, at a temperature of about 40° C. to 60° C., using a HBr and/or Cl2 as etch gases. Also, in the embodiments provided, the bias voltage used in the biased dry etching process may be tuned to allow better control of an etching direction to achieve desired profiles for the S/D recess regions.

Then, applying a wet etching process to the source and drain (S/D) recess regions using hydration tetramethyl ammonium (TMAH), the perpendicular sidewalls of the S/D recess regions are changed to a slope formed of (111) crystal plane of the substrate 202 to form the S/D recess cavities 220, 230, 240, and 250.

As a result of such etching processes, there may be formed a plurality of facets formed of the (111) crystal plane in each S/D recess cavities 220, 230, 240, and 250. It should be noted that periphery environment with or without an etch stop can influence resulted features of the S/D recess cavities 220, 230, 240, and 250. As one example, the S/D recess cavity 220 between the gate stack 210 a and isolation structure 206 a have respective sidewall surfaces each defined by a bottom surface 220 c and facets 220 a, 220 b, and 220 d due to the isolation structure 206 a acting as an etch stop. Thereby, the facet 220 a and facet 220 b thus formed intersect to each other and define together a wedge 220 w in the S/D recess cavity 220, such that the wedge-shaped S/D recess cavity 220 invades in the substrate 202 into the region right underneath the spacer 218 a toward the channel region. As another example, the S/D recess cavity 230 between the adjacent gate stacks 210 a, 210 b without an etch stop have respective sidewall surfaces each defined by a bottom surface 230 c and facets 230 a, 230 b, 230 d, and 230 e. Thereby, the facet 230 d and facet 230 e thus formed intersect to each other and define together a wedge 230 w in the S/D recess cavity 230, such that the wedge-shaped S/D recess cavity 230 invades in the substrate 202 into the region right underneath the spacer 218 b toward the channel region.

In the illustrated example, the bottom surface 220 c is formed of (100) crystal plane of the substrate 202 parallel to the major surface 202 s of the substrate 202, while the facet 220 a forms an angle θ₁ to the bottom surface 220 c. Further, the facets 220 b forms a smaller angle θ₂ than the angle θ₁ with respect to the bottom surface 220 c. In the structure of FIG. 3, the angle θ₁ takes the range of 90-150 degrees, while the angle θ₂ takes the range of 40-60 degrees. In this case, too, the angles take the values of 146 degrees and 56 degrees respectively in the case the facets 220 a, 220 b are formed by the (111) crystal plane of the substrate 202. However, it should be noted that the structure of FIG. 3 is not limited in the case in which the facets 220 a, 220 b are formed by the (111) crystal plane.

Further, the bottom surface 220 c is formed at a depth D₁ as measured from the major surface 202 s of the substrate 202, while the facet 220 a is formed down to a depth D₂. In the structure of FIG. 3, the depth D₁ is the range of 20 to 70 nm, while the depth D₂ is the range of 5 to 60 nm. By optimizing the depth D₂ and a distance between the mutually opposing wedges 220 w, 230 w, it becomes possible to confine the uni-axial compressive stress of a strained material 222 (shown in FIGS. 5A and 5B) to the channel region effectively, thereby enhancing the device performance.

The process steps up to this point have provided the substrate 202 having the S/D recess cavities 220, 230, 240, 250 adjacent to the gate stacks 210 a, 210 b, and 210 c. Referring to FIGS. 1, 4, 5A and 5B, the method 100 continues with step 108 in which a strained material 222 is selectively grown in the S/D recess cavities 220, 230, 240, 250 in the substrate 202 using an LPCVD process. Because the lattice constant of the strained material 222 is different from the substrate 202, the channel region of the semiconductor device 200 is strained or stressed to enable carrier mobility of the device and enhance the device performance.

In the present embodiment, a pre-cleaning process may be performed to clean the S/D recess cavities 220, 230, 240, 250 with hydrofluoric acid (HF) or other suitable solution. Then, a lower portion 222 l of a strained material 222 such as silicon germanium (SiGe) is selectively grown by an LPCVD process to partially fill the S/D recess cavities 220, 230, 240, 250 in the substrate 202. In one embodiment, the LPCVD process is performed at a temperature of about 400 to 800° C. and under a pressure of about 1 to 15 Torr, using SiH₂Cl₂, HCl, GeH₄, B₂H₆, and H₂ as reaction gases, wherein a ratio of a mass flow rate of the SiH₂Cl₂ to a mass flow rate of the HCl is in the range of about 0.45 to 0.55. Thermodynamically, growth rate of the close-packed (111) crystal plane of the strained material 222 is much greater than growth rates of other crystal planes of the strained material 222. In one embodiment, the strained material 222 is therefore grown from the facets 230 a, 230 b, 230 d, and 230 e to center of the S/D recess cavities 230 in the substrate 202. In another embodiment, the strained material 222 is therefore grown from the facets 220 a, 220 b, and 220 d to center of the S/D recess cavities 220 in the substrate 202.

Basically, a dielectric material with an amorphous structure fails to offer homogeneous nucleation sites to grow an epitaxial material. In the present embodiment, both the spacer 218 a and the isolation structure 206 a are dielectric materials, so that the growth of the strained material 222 in the S/D recess cavity 220 will terminate while a (111) crystal plane of the strained material 222 intersects the outer point 218 p at the bottom surface 218 s of the spacer 218 a and extend downward to the isolation structure 206 a. However, further growth of the strained material 222 in the S/D recess cavity 230 occurs at the same time if the LPCVD process continues.

It can be seen in FIG. 4 that the lower portion 222 l of the strained material 222 in the S/D recess cavity 220 occupies a small portion of the S/D recess cavity 220 and thus can not deliver a given amount of strain into channel region of the semiconductor device 200, thereby increasing the likelihood of device instability and/or device failure. A method to overcome growth energy barrier of the strained material 222 in the S/D recess cavity 220 for further growth of the strained material 222 in the S/D recess cavity 220 is hereafter provided according to various aspects of the present disclosure.

In the present embodiment, the LPCVD process is then changed and performed at a temperature of about 660 to 700° C. and under a pressure of about 13 to 50 Torr, using SiH₂Cl₂, HCl, GeH₄, B₂H₆, and H₂ as reaction gases, to form an upper portion 222 u of the strained material 222, wherein a ratio of a mass flow rate of the SiH₂Cl₂ to a mass flow rate of the GeH₄ is in the range of about 10 to 50, and a ratio of a mass flow rate of the SiH₂Cl₂ to a mass flow rate of the HCl is in the range of about 0.8 to 1.5. Because less HCl mass flow rate at this step providing less etching chemical during selectively epitaxial process makes isotropic growth of the upper portion 222 u of the strained material 222 easier, other crystal planes except the close-packed (111) crystal plane have more opportunity to appear to form the upper portion 222 u (shown in FIG. 5A). In another embodiment, the growth step of the lower portion 222 l of the strained material 222 can be replaced by this growth step. In other words, the strained material 222 can be formed using one growth step. Accordingly, the disclosed method of fabricating a semiconductor device 200 may fabricate a large-volume strained structure to enhance carrier mobility and upgrade the device performance.

In summary, the strained material 222 below the major surface 202 s of the substrate 202 disposed between the spacer 218 a and the isolation structure 206 a comprises the upper portion 222 u and the lower portion 222 l separated by a transition plane 222 p at an acute angle θ₃ to the major surface 202 s of the substrate 202, wherein the transition plane 222 p intersects the outer point 218 p and extends downward to the outer edge 206 e of the isolation structure 206 a, wherein the upper portion 222 u comprises a first facet 222 a adjacent to the spacer 218 a (shown in FIG. 5A). In one embodiment, the acute angle θ₃ takes the range of 50-60 degrees. In another embodiment, the acute angle θ₃ is at the range of 53-59 degrees. In the present embodiment, the acute angle θ₃ is 56 degrees and the transition plane 222 p is defined by the (111) crystal plane of the strained material 222.

It is observed that an acute angle θ₄ of the first facet 222 a to the major surface 202 s of the substrate 202 is less than the acute angle θ₃ of the transition plane 222 p to the major surface 202 s of the substrate 202. In one embodiment, the first facet 222 a is defined by the (311) crystal plane of the strained material 222. In the present embodiment, the upper portion 222 u further comprises a second facet 222 b separated from the spacer 218 a by the first facet 222 a, wherein the acute angle θ₄ of the first facet to the major surface 202 s of the substrate 202 is less than an acute angle θ₅ of the second facet 222 b to the major surface 202 s of the substrate 202. In one embodiment, the second facet 222 b is defined by the (111) crystal plane of the strained material 222. Further, the first facet 222 a and second facet 222 b are planar surfaces. However, it should be noted that the structure of FIG. 5A is not limited in the case in which the additional facets of the strained material 222 may also be formed due to isotropic growth of the upper portion 222 u.

Further, a cross-sectional area of the lower portion 222 l is greater than a cross-sectional area of the upper portion 222 u. In some embodiments, this is enough to deliver a given amount of strain into channel region of the semiconductor device 200. Accordingly, the disclosed method of fabricating a semiconductor device 200 may fabricate a strained structure to enhance carrier mobility and upgrade the device performance.

In some embodiments, further growth of the upper portion 222 u of the strained material 222 until the cross-sectional area of the lower portion 222 l less than a cross-sectional area of the upper portion 222 u (shown in FIG. 5B) is desired to deliver a given amount of strain into channel region of the semiconductor device 200.

In some embodiments, selective growth of an additional strained material 232 in the S/D recess cavity 230 separated from the strained material 222 in the S/D recess cavity 220 by the gate stack 210 a is not confined by the isolation structure 206 a, growth rate of the additional strained material 232 is thus greater than the growth rate of the strained material 222 in the S/D recess cavity 220. Because both the materials 222, 232 are grown simultaneously, a cross-sectional area of the additional strained material 232 is greater than a cross-sectional area of the strained material 222.

In one embodiment, the selective growth of the additional strained material 232 is terminated while the material 232 is a distance ranging from about 10 to 100 nm below the surface 202 a of the substrate 202 (not shown). In another embodiment, the selective growth of the additional strained material 232 continues until the material 232 extends a distance ranging from about 10 to 100 nm above the major surface 202 a of the substrate 202. Further growth of the additional strained material 232 may occupy space required by low-resistance silicide formed on the material 232, thereby increasing short opportunity between adjacent devices. Therefore, the strained material 222 on the structure of FIG. 5B is limited below the major surface 202 s of the substrate 202 disposed between the spacer 218 a and the isolation structure 206 a due to confined growth of the additional strained material 232.

In summary, the strained material 222 below the major surface 202 s of the substrate 202 disposed between the spacer 218 a and the isolation structure 206 a comprises the upper portion 222 u and the lower portion 222 l separated by a transition plane 222 p at an acute angle θ₃ to the major surface 202 s of the substrate 202, wherein the transition plane 222 p intersects the outer point 218 p and extends downward to the outer edge 206 e of the isolation structure 206 a, wherein the upper portion 222 u comprises a first facet 222 a adjacent to the spacer 218 a (shown in FIG. 5B). In one embodiment, the acute angle θ₃ takes the range of 50-60 degrees. In another embodiment, the acute angle θ₃ is at the range of 53-59 degrees. In the present embodiment, the acute angle θ₃ is 56 degrees and the transition plane 222 p is defined by the (111) crystal plane of the strained material 222.

An acute angle θ₄ of the first facet 222 a to the major surface 202 s of the substrate 202 is less than the acute angle θ₃ of the transition plane 222 p to the major surface 202 s of the substrate 202. In one embodiment, the first facet is defined by the (311) crystal plane of the strained material 222. In the present embodiment, the upper portion 222 u further comprises a second facet 222 b separated from the spacer 218 a by the first facet 222 a, wherein the acute angle θ₄ of the first facet to the major surface 202 s of the substrate 202 is less than an acute angle θ₅ of the second facet 222 b to the major surface 202 s of the substrate 202. In one embodiment, the second facet 222 b is defined by the (111) crystal plane of the strained material 222. Further, the first facet 222 a and second facet 222 b are planar surfaces. However, it should be noted that the structure of FIG. 5B is not limited in the case in which the additional facets of the strained material 222 may also be formed due to isotropic growth of the upper portion 222 u. Accordingly, the disclosed method of fabricating a semiconductor device 200 may fabricate a strained structure to enhance carrier mobility and upgrade the device performance.

Further, a maximum thickness 222 t of the strained material 222 on the structure of FIGS. 5A and 5B is in the range of about 20 to 300 nm. The strained material 222 comprises SiGe having a Ge concentration in the range of about 10% to 40% in terms of atomic percent.

It is understood that the semiconductor device 200 may undergo further CMOS processes to form various features such as contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. In some embodiment, the gate stacks 210 a, 210 b, 210 c may be dummy gate stacks. Thus, the CMOS processes further comprise a “gate last” process to replace the poly-silicon gate electrode with a metal gate electrode to improve device performance. In one embodiment, the metal gate electrode may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other suitable conductive materials, or combinations thereof. It has been observed that the modified strained structure provides a given amount of strain into channel region of a semiconductor device, thereby enhancing the device performance.

In one embodiment, a semiconductor device comprises a substrate comprising a major surface; a gate stack comprising a sidewall over the substrate; a spacer over the substrate adjoining the sidewall of the gate stack, wherein the spacer comprises a bottom surface having an outer point that is the point on the bottom surface farthest from the gate stack; an isolation structure in the substrate on one side of the gate stack having an outer edge closest to the spacer; and a strained material below the major surface of the substrate disposed between the spacer and the isolation structure comprising an upper portion and a lower portion separated by a transition plane at an acute angle to the major surface of the substrate, wherein the transition plane intersects the outer point and extends downward to the outer edge of the isolation structure, wherein the upper portion comprises a first facet adjacent to the spacer, wherein an acute angle of the first facet to the major surface of the substrate is less than the acute angle of the transition plane to the major surface of the substrate.

In another embodiment, a semiconductor device comprises a substrate comprising a major surface; a gate stack comprising a sidewall over the substrate; a spacer over the substrate adjoining the sidewall of the gate stack, wherein the spacer comprises a bottom surface having an outer point that is the point on the bottom surface farthest from the gate stack; an isolation structure in the substrate on one side of the gate stack having an outer edge closest to the spacer; a strained material below the major surface of the substrate disposed between the spacer and the isolation structure comprising an upper portion and a lower portion separated by a transition plane at an acute angle to the major surface of the substrate, wherein the transition plane intersects the outer point and extends downward to the outer edge of the isolation structure, wherein the upper portion comprises a first facet adjacent to the spacer, wherein an acute angle of the first facet to the major surface of the substrate is less than the acute angle of the transition plane to the major surface of the substrate; and an additional strained material separated from the strained material by the gate stack, wherein a cross-sectional area of the additional strained material is greater than a cross-sectional area of the strained material.

In still another embodiment, a method for fabricating a semiconductor device comprises providing a substrate comprising a major surface; forming a gate stack over the major surface of the substrate; recessing the substrate to form source and drain recess cavities adjacent to the gate stack in the substrate; and selectively growing a strained material in the source and drain recess cavities in the substrate using an LPCVD process, wherein the LPCVD process is performed at a temperature of about 660 to 700° C. and under a pressure of about 13 to 50 Torr, using SiH₂Cl₂, HCl, GeH₄, B₂H₆, and H₂ as reaction gases.

While the disclosure has described by way of example and in terms of the preferred embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A semiconductor device comprising: a substrate comprising a major surface; a gate stack comprising a sidewall over the substrate; a spacer over the substrate adjoining the sidewall of the gate stack, wherein the spacer comprises a bottom surface having an outer point that is farthest from the gate stack; an isolation structure in the substrate on one side of the gate stack having an outer edge closest to the spacer; and a strained material below the major surface of the substrate disposed between the spacer and the isolation structure comprising an upper portion and a lower portion separated by a transition plane at an acute angle to the major surface of the substrate, wherein the transition plane intersects the outer point and extends downward to the outer edge of the isolation structure, and a cross-sectional area of the lower portion is greater than a cross-sectional area of the upper portion, wherein the upper portion comprises a first facet adjacent to the spacer, wherein an acute angle of the first facet to the major surface of the substrate is less than the acute angle of the transition plane to the major surface of the substrate.
 2. The semiconductor device of claim 1, wherein the strained material comprises SiGe.
 3. The semiconductor device of claim 2, wherein a Ge concentration of the SiGe is in the range of about 10% to 40% in terms of atomic percent.
 4. The semiconductor device of claim 1, wherein a maximum thickness of the strained material is in the range of from about 20 to 300 nm.
 5. The semiconductor device of claim 1, wherein the transition plane is defined by a (111) crystal plane of the strained material.
 6. The semiconductor device of claim 1, wherein the first facet is defined by a (311) crystal plane of the strained material.
 7. The semiconductor device of claim 1, wherein the upper portion further comprises a second facet separated from the spacer by the first facet, wherein the acute angle of the first facet to the major surface of the substrate is less than an acute angle of the second facet to the major surface of the substrate.
 8. The semiconductor device of claim 7, wherein the first facet and second facet are planar surfaces.
 9. The semiconductor device of claim 1, wherein a cross-sectional area of the lower portion is less than a cross-sectional area of the upper portion.
 10. The semiconductor device of claim 1 further comprises an additional strained material separated from the strained material by the gate stack, wherein a cross-sectional area of the additional strained material is greater than a cross-sectional area of the strained material.
 11. The semiconductor device of claim 10, wherein the additional strained material is below the major surface of the substrate.
 12. The semiconductor device of claim 10, wherein at least a portion of the additional strained material is above the major surface of the substrate.
 13. The semiconductor device of claim 1, wherein at least a portion of a lattice orientation of the upper portion is different than a lattice structure of the lower portion.
 14. A semiconductor device comprising: a substrate comprising a major surface; a gate stack comprising a sidewall over the major surface; a spacer over the substrate adjoining the sidewall of the gate stack, wherein the spacer comprises a bottom surface having an outer point that is farthest from the gate stack; an isolation structure in the substrate on at least one side of the gate stack having an outer edge closest to the spacer; and a strained material below the major surface, the strained material disposed between the spacer and the isolation structure comprising an upper portion and a lower portion separated by a transition plane, wherein the transition plane intersects the outer point and extends downward to the outer edge of the isolation structure, wherein the lower portion extends beneath the spacer, the lower portion includes a bottom surface parallel to the major surface, a first facet forming an obtuse angle with the bottom surface, and a second facet forming an acute angle with the bottom surface, and the upper portion comprises a third facet adjacent to the spacer.
 15. The semiconductor device of claim 14, wherein the transition plane is at an acute angle to the major surface of the substrate, wherein an acute angle of the third facet to the major surface of the substrate is less than the acute angle of the transition plane to the major surface of the substrate.
 16. The semiconductor device of claim 14, wherein the first facet extends to a first depth below the major surface, the first depth ranging from 5 nanometers (nm) to 60 nm.
 17. The semiconductor device of claim 14, wherein the bottom surface extends to a total depth below the major surface, the total depth ranging from 20 nm to 70 nm.
 18. A semiconductor device comprising: a substrate comprising a major surface; a gate stack comprising a sidewall over the major surface; a spacer over the substrate adjoining the sidewall of the gate stack, wherein the spacer comprises a bottom surface having an outer point that is farthest from the gate stack; an isolation structure in the substrate on at least one side of the gate stack having an outer edge closest to the spacer; and a strained material completely below the major surface, the strained material disposed between the spacer and the isolation structure comprising an upper portion and a lower portion separated by a transition plane, wherein the transition plane intersects the outer point and extends downward to the outer edge of the isolation structure, wherein the lower portion extends beneath the spacer, the lower portion includes a bottom surface parallel to the major surface, a first facet extending to a first depth below the major surface forming a first angle with the bottom surface, and a second facet extending to a second depth below the major surface and forming a second angle different than the first angle with the bottom surface.
 19. The semiconductor device of claim 18, wherein the transition plane is at an acute angle to the major surface of the substrate, and the upper portion comprises a third facet adjacent to the spacer, wherein an acute angle of the third facet to the major surface of the substrate is less than the acute angle of the transition plane to the major surface of the substrate.
 20. The semiconductor device of claim 18, wherein the first angle ranges from 90-degrees to 150-degrees, and the second angle ranges from 40-degrees to 60-degrees. 